1. The Field of the Invention
The present invention relates generally to water heaters, and more specifically, to methods for a control system used with fluidly and functionally connected water heaters.
2. Background Art
The art of using water heaters in cascaded fashion to meet large water heating loads is not new. In a cascade system, a plurality of water heaters is used to participate in sharing water heating load to meet a demand. Typically, in a large commercial building, apartment complex, hotel or laundromat, the demand for hot water can range from zero to a very large demand in an instant. Therefore, a system capable of providing a demand in real time ranging from very small to very large is needed.
A single commercial or residential hot water heater is incapable of providing such a wide ranging demand in real time. Another drawback of using a single unit under such circumstance is that it provides a single point of failure. When a single water heater is removed for repair or maintenance, the entire building would be without hot water. Other drawbacks of using a single water heater include the excessive physical size and inefficient heating associated with excessive physical size. One solution commonly used in solving the drawbacks associated with using a single heater for a severely varying and large demand is to leverage the heating capacity of multiple water heaters. As such, multiple water heater units may be cascaded to be fluidly and functionally connected to form a network of water heaters so that one water heater can be turned on to service a small demand while multiple units can be turned on simultaneously to service a sudden change to a large demand. Furthermore, a cascaded system involving multiple water heaters affords failure redundancy not available in a single water heater system. One or multiple units may be removed for service without interrupting the operation of remaining water heaters in the network.
In a conventional cascade system, the last water heater turned on is the modulating boiler while the capacity of other previously selected and turned on water heaters is pegged at their maximum output. As such, the last water heater turned on may experience excessive cycling on and off if a requested demand falls within a dead band. Various control schemes have been devised to provide for those situations where the overall heat demand for the system of a plurality of water heaters falls within a zone lying between the maximum heat output of a water heater and the sum of the maximum heat output of this water heater and the minimum heat output of the next adjacent water heater. This zone, which may be referred to as a dead band or dead zone, presents unique operational problems because the next adjacent boiler cannot modulate within that range.
Published US patent application 2008/0216771 entitled “CONTROL SYSTEM FOR MODULATING WATER HEATER” discloses a control system which minimizes the cycling on and off of such next adjacent boiler if the overall demand falls in a dead band. The control system is claimed to be particularly suited for use with a plurality of modulating water heaters, which may be boilers, arranged for control in a cascade sequence where a first boiler is brought online at its firing point and is then continuously modulated up to its maximum output, and then, the first boiler is maintained at its constant output while firing a second boiler which is then modulated from its firing point up to its maximum output as the overall heat demand on the system increases. In a similar manner, each boiler is brought up to its maximum output before the next adjacent boiler is fired, and all previously fired boilers are maintained at maximum output with the modulation for the system coming from modulation of the last fired boiler. While heater cycling is minimized, the '771 application falls short of addressing the issue of distributing flow to conserve energy.
In conventional cascade water heater systems, there often exists a significant disparity in usage between water heaters in a network. The order in which water heaters are turned on is fixed. A small demand causes a first water heater to turn on. As demand increases, more water heaters are turned on. As a result, the water heaters arranged to turn on first experience significantly higher accumulated usage than others, especially ones serving low demands. Water heaters experiencing higher accumulated usage require more regular preventative or unscheduled maintenance while others are underutilized. One attempt to solve such a problem is evidenced in water heaters marketed under the trade name “Eternal Advanced Hybrid Water Heating” by Grand Hall Enterprise, Ltd. The operator's manual labeled 157110293 and dated Jul. 4, 2009 introduces a host and sub concept in which a host unit is selected as the first unit to fire when demand for hot water is detected and it control multiple sub units. According to “Specifications and Features” (page 3) and “MCU Operational Sequence Flow Chart” (page 8) sections of this operator's manual, the designation of a water heater controller as the host is changed every 24 hours in order to distribute wear and tear across all units in a networked system.
Published US patent application 2008/0216771 further discloses a scheme in which each boiler includes a controller and may serve as a lead boiler and its controller as the master controller. The role of lead boiler is periodically rotated between each of the boilers in the system so as to substantially equalize the number of operating hours experienced by each boiler. The practice of using operating hours alone as a measure to estimate a water heater's remaining life is fraught with uncertainties since there are other significant factors affecting the water heater's remaining life. In use, a water heater delivers an amount of hot water at a temperature over a period of time. Given a fixed number of operating hours, the damage done to a water heater used to deliver water at 140 degrees Fahrenheit is substantially different than the damage done to a water heater used to deliver water at 102 degrees Fahrenheit. Based on this premise, the applicants believe that there needs to be an improved or more accurate method of estimating remaining life to efficiently control water heaters in a networked or cascaded system.